A conventional HST circuit for traveling a wheel loader will be described, the HST circuit being configured as a closed circuit having a hydraulic pump and a hydraulic motor (see, e.g., Patent Citation 1). Referring to FIG. 8, a hydraulic pump 1 is a variable capacity bidirectionally-rotatable pump to be driven by a drive mechanism 2 such as an engine. In the case of performing forward rotation F of the hydraulic pump 1, oil discharged from the hydraulic pump 1 flows into a bidirectionally-rotatable hydraulic motor 4 through a line 3, and then, flows out to a line 5. The oil having passed through the line 5 flows into the hydraulic pump 1. Forward rotation F of the hydraulic motor 4 occurs due to a pressure difference between before and after the hydraulic motor 4.
In the case of performing reverse rotation R of the hydraulic pump 1, the oil discharged from the hydraulic pump 1 flows into the hydraulic motor 4 through the line 5, and then, flows out to the line 3. The oil having passed through the line 3 flows into the hydraulic pump 1. Reverse rotation R of the hydraulic motor 4 occurs due to the pressure difference between before and after the hydraulic motor 4.
Relief valves 6, 7 are further installed in the circuit. These relief valves 6, 7 are set to a predetermined relief pressure. When a high pressure generated at the line 3 reaches the relief pressure, the relief valve 6 is operated such that the oil flows out to a low-pressure line 8, thereby preventing overload of the line 3. Similarly, a high pressure generated at the line 5 reaches the relief pressure, the relief valve 7 is operated such that the oil flows out to a low-pressure line 9, and in this manner, overload of the line 5 is prevented.
The hydraulic pump 1 is configured so that the amount of oil discharged from the hydraulic pump 1 can be controlled from 0 (zero) to the maximum value by changing the tilting amount of a swash plate 1a from 0 (zero) to the maximum value in accordance with an external signal (not shown).
Referring to FIG. 9, there has been proposed another HST circuit used for travelling a vehicle (also referred to as a travelling HST circuit), which is configured for effectively utilizing high-pressure oil such that an accumulator 10A is connected to a hydraulic motor, that high-pressure oil generated from the hydraulic motor upon deceleration of a vehicle is accumulated in the accumulator 10A, and that the high-pressure oil accumulated in the accumulator 10A is used upon start of the vehicle (see Patent Citation 2).